(i) Field of the Invention:
This invention relates to a process for concentration of an aqueous solution of amino acid and particularly, to a process for concentrating amino acids having a low solubility in water at isoelectric point by means of semi-permeable membranes.
(ii) Description of the Prior Art:
Reverse osmosis membranes (hereinafter referred to as "RO membranes") are applicable to the recovery and concentration of end materials in a solution at temperatures near to room temperature without bringing about a phase change.
The concentration process using RO membranes which has many advantages such as less energy consumption is recently attracting attention and finding its uses in a variety of fields. A typical example of the use is the separation of extremely low molecular weight solutes exemplified by inorganic salts. The recent diversified uses owe to the development of composite RO membranes having a good heat and solvent resistance and "loose" RO membranes resembling ultrafiltration membranes which permit the passage of extremely low molecular weight solutes.
A prospective application area of RO membranes is the separation and purification of amino acids from dilute aqueous solutions produced by fermentation, chemical synthesis, or enzymatic synthesis, which is carried out by crystallization, ion-exchange, electrodialysis, or solvent extraction at the present time.
In these processes for concentration there are, in many cases, used means consuming a great deal of heat energy such as concentration method by heat evaporation and multiple effect evaporators. Accordingly, if the concentration process using the RO membrane is applicable to the concentration of an aqueous solution of amino acid, it can be highly expected as a concentration process of less energy consumption.
It is expected that the RO membrane, which can be applied to solutions containing no suspending solids owing to the principle, will be able to concentrate a dilute aqueous solution of an amino acid, such as proline, arginine, lysine, sodium glutamate, serine, and glycine which are all highly soluble in water, up to high concentrations even at the isoelectric point of the amino acids. In actual, a working example of such use of RO membranes is reported in, for example, Japanese Patent Laid-open No. 152355/1984. However, RO membranes cannot concentrate to high concentrations the aqueous solutions of aromatic ring-containing amino acids such as tryptophan, tyrosine, and phenylalanine and sulfur-containing amino acids such as cystine and methionine, which are sparingly soluble in water at the isoelectric point. This is because the RO membrane is clogged with the amino acid which separates out upon saturation.
Incidentally, an amino acid is an amphoteric electrolyte which has a carboxyl group (--COOH) and an amino group (--NH.sub.2) in the same molecule. Each amino acid has its characteristic constants which are the acid dissociation constants K.sub.1 and K.sub.2. Therefore, an amino acid dissociates more as the pH of an aqueous solution of the amino acid is adjusted to pK.sub.1 or pK.sub.2 from the isoelectric point, and the saturation solubility of the amino acid greatly increases in the pH range lower than pK.sub.1 or higher than pK.sub.2. Amino acids having a low solubility in water as mentioned above are neutral amino acids and the pH of isoelectric point is 5 to 6. Therefore, for example in case of tryptophane (pK.sub.1 =2.3, pK.sub.2 =9.39), in order to realize a solubility of 8 to 10 wt. % only by adjustment of pH, the pH must be set in the range of not less than 11 or not more than 2 which is the pH condition deviating extensively from the pK.sub.1 and pK.sub.2 values. In this way, when the pH is adjusted to the range of less than pK.sub.1 of said amino acid or of more than pK.sub.2, a large amount of a pH adjusting agent is not only required, but also a pH of the resulting aqueous solution shows a strong acidity or strong alkalinity which deviates from the tolerable pH range of RO membrane, so that there are raised problems in durability of the membrane and in corrosion resistance of a concentration apparatus using the RO membrane Thus, it is, in practice, difficult to increase the solubility of the amino acid by adjustment of a pH only thereby effecting a high concentration through membranes.
Also, there is proposed a method of increasing a saturation solubility by raising temperature thereby effecting a high concentration through membranes. However, there is a limit to the raising of temperature because amino acids are not so good in thermal stability, and the solubility of amino acids is not so temperature-dependent. In case of tryptophan, for instance, the solubility increases by 2% at the best when the solution temperature is raised to 60.degree. C. Thus, the raising of solution temperatures will not provide a high degree of concentration.
Further, most RO membranes of practical use do not withstand temperatures higher than 50.degree. C., although those of heat-resistant type are being developed recently. At high temperatures, they are subject to unreversible deterioration such as change in quality, which shortens the service life.
Thus, the method of increasing the solubility by elevating temperature for concentration through membranes has some problems as mentioned above and therefore, cannot be an effective means for the concentration of amino acids having a low solubility at the isoelectric point and containing hydrophobicity.